For this conversion much depends on the engine and how big the frame or boiler dia. is. My Thunderbolt/Lion, ex the late Bruce Ward, is converted from 5" gage to 4 3/4" gage. The boiler dia. just made it possible. I do not know exactly what was done in the conversion but it worked. The splashers will not fit anymore. Probably the most challenging thing was probably the need to make a new main driving axle which contains all the valve eccentrics and inside piston throws. The engine runs very well. You would do well to purchase a set of the drawings for the particular engine you want to convert. Now if you have already purchased the engine...........
Cary